Sputtering is a phenomenon that atoms or molecules are projected from a target, against which an inactive material such as Ar, etc. impinges at a high speed, and that the projected atoms or molecules are attached to a substrate to form a thin film. A magnetron sputtering method uses a magnetic field in a cathode to accelerate an accumulating speed of a target material on a substrate, forming a thin film at a low temperature because of no collision of electrons to the substrate. Accordingly, the magnetron sputtering method is widely used to form thin films on substrates in the production of electronic devices such as semiconductor ICs, flat panel displays and solar cells, reflection films, etc.
A magnetron sputtering apparatus comprises, in a vacuum chamber, a substrate (anode), a target (cathode) opposing the substrate, and a magnetic-field-generating apparatus arranged under the target. Voltage applied between the anode and the cathode causes glow discharge, ionizing an inert gas (for example, Ar gas at about 0.1 Pa) in the vacuum chamber, and secondary electrons discharged from the target are captured by a magnetic field generated by the magnetic-field-generating apparatus to cause a cycloidal motion on the target surface. The cycloidal motion of electrons accelerates the ionization of gas molecules, resulting in a drastically increased film-forming speed than without a magnetic field, with larger film adhesion strength.
A magnetic-field-generating apparatus in a magnetron sputtering apparatus, which can generate a circular or racetrack-shaped magnetic field, conventionally comprises, as shown in FIGS. 10(a) to 10(c), for example, a straight center magnetic pole member, a peripheral magnetic pole member surrounding the center magnetic pole member, and pluralities of permanent magnets arranged between the center magnetic pole member and the peripheral magnetic pole member for a horizontal magnetization direction with magnetic poles of the same polarity opposing the center magnetic pole member. When magnetron sputtering is conducted using a magnetic-field-generating apparatus having such a structure, the target undergoes V-cross-sectioned erosion as shown in FIG. 11, resulting in insufficient use efficiency of the target.
In conventional magnetron sputtering, to improve the use efficiency of a target, means for making an erosion region of a target more uniform, namely means for making the cross section shape of erosion from a V shape to a U shape have been contemplated. A more uniform erosion region of a target can be achieved by, for example, the mechanical swinging of a target or a magnetic circuit, the movement or rotation of part of magnets in the magnetic circuit, etc. However, the mechanical swinging of a target or a magnetic circuit for achieving uniform erosion of the target results in drastic cost increase such as a larger apparatus, so that it is practically difficult.
JP 2004-83974 A discloses in FIGS. 1 and 2 a magnetic-field-generating apparatus for forming a film by sputtering, which generates a magnetic field having magnetic force lines, in which positions at which a vertical component of a magnetic flux density is zero on a substrate-side target surface expand outward from a center portion of the target as vertically separating from the target, thereby suppressing the diffusion of plasma toward the substrate to provide uniform distributions of film thickness and quality. JP 2004-83974 A describes that by forming magnetic force lines near the target surface substantially in parallel with the target surface, an erosion region can be expanded wider in a range from a target center portion toward its peripheral portion, thereby improving the use efficiency of a target material.
However, the magnetic-field-generating apparatus of JP 2004-83974 A forms magnetic force lines providing a magnetic tunnel between center and peripheral portions of the target on the target surface, and a ratio B1/B2 being 2.5 or more, wherein B1 is an absolute value of a vertical component of a magnetic flux density at a position at which a horizontal component of a magnetic flux density is zero near the center portion of the target surface, and B2 is the maximum absolute value of a vertical component of a magnetic flux density near the peripheral portion of the target surface, namely, the vertical component B1 of a magnetic flux density in the center portion being as large as 2.5 times or more the vertical component B2 of a magnetic flux density in the peripheral portion, resulting in insufficient uniformity of an erosion region. Thus, further improvement is desired.